In the current oil-driven economy, vast amounts of petroleum fuels are stored in holding tanks, the majority of such tanks being located underground. These installations provide an economic alternative to aboveground installations and result in a significant reduction in the risk of fire or explosions. Underground storage tanks installed prior to 1985 were usually constructed of steel with only minimal protection from corrosion. Furthermore, few of these tanks were fitted with means for detecting leaks of stored fuel. Unfortunately, these factors have contributed to the undetected contamination of adjacent soil and aquifer.
In response to the problems created by leakage of such underground storage tanks, the United States Environmental Protection Agency recently sponsored legislation, placing financial responsibility for any such leakage upon the owners and operators of such tanks. Initially, underground storage tank owners and operators were required to register commercial tanks, so providing an index as to the number and size of the tanks present in the United States. Approximately 3,000,000 tanks were registered in response to this requirement. Thereafter, the Agency published additional regulations which required retrofitting commercial tanks with release monitoring systems including a means for the detection of hydrocarbon fuel seepage into the environment and also requiring the maintenance of liability insurance in the event of actual leakage.
Accordingly, there has been widespread interest in the industrial community for the development of suitable detectors capable of sensing leakage of liquid hydrocarbon fuels from underground storage tanks. Among the many industries feeling the impact of such regulations is the telephone industry which collectively maintains in excess of 20,000 storage tanks which are used to store diesel fuel for central office emergency power, fuel oil to heat buildings and gasoline for vehicle fleets. Although some of these tanks are currently exempt from the Federal requirements, it is anticipated that legislation at both the Federal and State levels will eventually require leak detection equipment on all such tanks. Additionally, hydrocarbon fuels such as gasoline, kerosene or diesel fuel present in the soil or floating on the water table can infiltrate underground telephone cable ducts and drain into utility holes. Failure to promptly remove these contaminants may result in irreversible damage to the seals on exposed cables or splice cases, so permitting water to enter the cable core or splice case, which results in the short circuiting of the conductors and necessitates expensive repair or replacement.
Heretofore, a wide variety of leak detectors have been proposed for this purpose. Among such detectors are mechanical float devices, thermal conductivity detectors, metal oxide semiconductors and conductive rubber materials. Each of these devices has certain advantages and limitations. However, the conductive rubbers are potentially the most attractive because they are generally durable, relatively inexpensive and exhibit a significant increase in resistance in the presence of non-polar organic materials such as the hydrocarbon fuels. In practice, however, they have not emerged as the optimum underground fuel sensor design because, up to now, their disadvantages have outweighed their advantages.
All of these conductive rubber detectors are based upon insulating plastic or rubber materials which comprise conductive bodies, such as carbon black or metallic particles, sufficient to yield a semiconductive composite. Upon contact with hydrocarbon liquid and/or vapors, the resistance of the composite increases dramatically. Thus, for example, U.S. Pat. No. 2,691,134 issued to C. S. Ford on Oct. 5, 1954 relates to a device for detecting the leakage of inflammable fluids such as gasoline, gasoline vapors and the like from storage receptacles. The patentee describes a sensor comprising an electrically conductive rubber having an initial resistance which increases dramatically upon contact with hydrocarbon fuel. Upon removal of the fluid, the initial resistance of the conductive material is restored.
Following this invention, several carbon black/rubber composites were described in the patent literature, each optimizing a particular aspect of the aforementioned Ford patent. Thus, for example, U.S. Pat. No. 3,045,198 issued to J. P. Dolan and William N. Jordan on Jul. 17, 1962 relates to a detection device based upon the concept of adsorption of hydrocarbon vapors on exposed carbon black particles adhered to a rigid substrate with a silicone rubber adhesive. In the presence of hydrocarbon vapors, the resistance of this device increases dramatically. Further modifications to this device were described in later patents issued to Dolan, namely, U.S. Pat. Nos. 4,129,030, 4,224,595, and 4,237,721. There are, however, practical drawbacks to the Dolan devices. Initially, it is noted that these devices are useful for sensing only hydrocarbon vapors and will not tolerate continuous contact with liquid hydrocarbons or water. Furthermore, these sensors are not rugged, cannot be submerged and require a rigid substrate. Accordingly, the Dolan sensors must be discarded after use and are useful only as point sensors, so limiting their applicability for commercial detection. Still further, the large carbon particles employed tend to fall out of the silicone adhesive with the passage of time, so causing the resistance to change.
An alternative device for detection of hydrocarbon fuels is described in U.S. Pat. No. 4,631,952 issued on Dec. 30, 1986 to L. F. Donaghey. This device comprises a conductive rubber sensor which is capable of detecting hydrocarbons in both the liquid and vapor state. The sensor includes spherical carbon particles dispersed homogeneously in a silicone rubber matrix. This composite is similar to the Dolan devices in that when contacted with liquid or gaseous hydrocarbons the resistance increases substantially presumably because of the swelling of the silicone matrix and/or adsorption of liquid on the carbon particle surface. In light of the fact that this composite is a homogeneous mixture of carbon particles and silicone, it is an enhancement of the Dolan device in that it functions in the presence of liquids and gases and is completely reversible. Additionally, it is easy to fabricate since dispersion of the conductive filler in the polymer is effected using conventional plastic or rubber mixing equipment. However, the Donaghey device does have two major limitations which makes it impractical as a device for detecting the presence of fuel in a variety of applications. The first limitation occurs because the sensor is so sensitive to hydrocarbon vapors that it continually gives rise to false alarms in areas where there may be no more than traces of hydrocarbon fuel spills even though they had been previously removed. Furthermore, many of the new tanks being installed underground are of the double wall variety. As such, the concentration of fuel vapor can be high in the annular space as a result of diffusion through fiberglass or seepage through welding points, so causing the device to trigger an alarm even though there was no liquid leak. The second limitation arises from the high carbon black concentration which causes a severe reduction in mechanical strength and thereby limits the use for distributed sensing applications.
Interestingly, one of the limitations of the Donaghey device was addressed in U.S. Pat. No. 4,855,706 issued to P. D. Hauptly on Aug. 8, 1989. The Hauptly device detects only liquid hydrocarbons and not interfering vapors. Analysis of the patent reveals that it is essentially the same as the Donaghey device but for the fact that the carbon concentration is much higher, namely, in the range of 78-97 weight percent. Due to the fact that the swellable polymer concentration is so small, only liquid hydrocarbons enlarge the material sufficiently to cause a significant increase in resistance. Unfortunately, this approach is unsatisfactory since the sensor material is very brittle, and like the Dolan devices, requires a rigid substrate with a defined shape. Thus, this device is only feasible for applications which utilize point sensing rather than distributed sensors.